/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.base.Predicates.equalTo;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.instanceOf;
import static com.google.common.base.Predicates.not;
import static com.google.common.collect.CollectPreconditions.checkRemove;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Objects;
import com.google.common.base.Optional;
import com.google.common.base.Preconditions;
import com.google.common.base.Predicate;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
import java.util.Queue;
import javax.annotation.Nullable;

/**
 * This class contains static utility methods that operate on or return objects of type
 * {@link Iterator}. Except as noted, each method has a corresponding {@link Iterable}-based method
 * in the {@link Iterables} class.
 *
 * <p>
 * <i>Performance notes:</i> Unless otherwise noted, all of the iterators produced in this class are
 * <i>lazy</i>, which means that they only advance the backing iteration when absolutely necessary.
 *
 * <p>
 * See the Guava User Guide section on
 * <a href= "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#iterables">
 * {@code Iterators}</a>.
 *
 * @author Kevin Bourrillion
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible(emulated = true)
public final class Iterators {
    private Iterators() {}

    /**
     * Returns the empty iterator.
     *
     * <p>
     * The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
     */
    static <T> UnmodifiableIterator<T> emptyIterator() {
        return emptyListIterator();
    }

    /**
     * Returns the empty iterator.
     *
     * <p>
     * The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
     */
    // Casting to any type is safe since there are no actual elements.
    @SuppressWarnings("unchecked")
    static <T> UnmodifiableListIterator<T> emptyListIterator() {
        return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
    }

    /**
     * This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
     * referenced by emptyModifiableIterator().
     */
    private enum EmptyModifiableIterator implements Iterator<Object> {
        INSTANCE;

        @Override
        public boolean hasNext() {
            return false;
        }

        @Override
        public Object next() {
            throw new NoSuchElementException();
        }

        @Override
        public void remove() {
            checkRemove(false);
        }
    }

    /**
     * Returns the empty {@code Iterator} that throws {@link IllegalStateException} instead of
     * {@link UnsupportedOperationException} on a call to {@link Iterator#remove()}.
     */
    // Casting to any type is safe since there are no actual elements.
    @SuppressWarnings("unchecked")
    static <T> Iterator<T> emptyModifiableIterator() {
        return (Iterator<T>) EmptyModifiableIterator.INSTANCE;
    }

    /** Returns an unmodifiable view of {@code iterator}. */
    public static <T> UnmodifiableIterator<T> unmodifiableIterator(final Iterator<? extends T> iterator) {
        checkNotNull(iterator);
        if (iterator instanceof UnmodifiableIterator) {
            @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
            UnmodifiableIterator<T> result = (UnmodifiableIterator<T>) iterator;
            return result;
        }
        return new UnmodifiableIterator<T>() {
            @Override
            public boolean hasNext() {
                return iterator.hasNext();
            }

            @Override
            public T next() {
                return iterator.next();
            }
        };
    }

    /**
     * Simply returns its argument.
     *
     * @deprecated no need to use this
     * @since 10.0
     */
    @Deprecated
    public static <T> UnmodifiableIterator<T> unmodifiableIterator(UnmodifiableIterator<T> iterator) {
        return checkNotNull(iterator);
    }

    /**
     * Returns the number of elements remaining in {@code iterator}. The iterator will be left
     * exhausted: its {@code hasNext()} method will return {@code false}.
     */
    public static int size(Iterator<?> iterator) {
        long count = 0L;
        while (iterator.hasNext()) {
            iterator.next();
            count++;
        }
        return Ints.saturatedCast(count);
    }

    /**
     * Returns {@code true} if {@code iterator} contains {@code element}.
     */
    public static boolean contains(Iterator<?> iterator, @Nullable Object element) {
        if (element == null) {
            while (iterator.hasNext()) {
                if (iterator.next() == null) {
                    return true;
                }
            }
        } else {
            while (iterator.hasNext()) {
                if (element.equals(iterator.next())) {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Traverses an iterator and removes every element that belongs to the provided collection. The
     * iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
     *
     * @param removeFrom the iterator to (potentially) remove elements from
     * @param elementsToRemove the elements to remove
     * @return {@code true} if any element was removed from {@code iterator}
     */
    @CanIgnoreReturnValue
    public static boolean removeAll(Iterator<?> removeFrom, Collection<?> elementsToRemove) {
        return removeIf(removeFrom, in(elementsToRemove));
    }

    /**
     * Removes every element that satisfies the provided predicate from the iterator. The iterator
     * will be left exhausted: its {@code hasNext()} method will return {@code false}.
     *
     * @param removeFrom the iterator to (potentially) remove elements from
     * @param predicate a predicate that determines whether an element should be removed
     * @return {@code true} if any elements were removed from the iterator
     * @since 2.0
     */
    @CanIgnoreReturnValue
    public static <T> boolean removeIf(Iterator<T> removeFrom, Predicate<? super T> predicate) {
        checkNotNull(predicate);
        boolean modified = false;
        while (removeFrom.hasNext()) {
            if (predicate.apply(removeFrom.next())) {
                removeFrom.remove();
                modified = true;
            }
        }
        return modified;
    }

    /**
     * Traverses an iterator and removes every element that does not belong to the provided
     * collection. The iterator will be left exhausted: its {@code hasNext()} method will return
     * {@code false}.
     *
     * @param removeFrom the iterator to (potentially) remove elements from
     * @param elementsToRetain the elements to retain
     * @return {@code true} if any element was removed from {@code iterator}
     */
    @CanIgnoreReturnValue
    public static boolean retainAll(Iterator<?> removeFrom, Collection<?> elementsToRetain) {
        return removeIf(removeFrom, not(in(elementsToRetain)));
    }

    /**
     * Determines whether two iterators contain equal elements in the same order. More specifically,
     * this method returns {@code true} if {@code iterator1} and {@code iterator2} contain the same
     * number of elements and every element of {@code iterator1} is equal to the corresponding
     * element of {@code iterator2}.
     *
     * <p>
     * Note that this will modify the supplied iterators, since they will have been advanced some
     * number of elements forward.
     */
    public static boolean elementsEqual(Iterator<?> iterator1, Iterator<?> iterator2) {
        while (iterator1.hasNext()) {
            if (!iterator2.hasNext()) {
                return false;
            }
            Object o1 = iterator1.next();
            Object o2 = iterator2.next();
            if (!Objects.equal(o1, o2)) {
                return false;
            }
        }
        return !iterator2.hasNext();
    }

    /**
     * Returns a string representation of {@code iterator}, with the format
     * {@code [e1, e2, ..., en]}. The iterator will be left exhausted: its {@code hasNext()} method
     * will return {@code false}.
     */
    public static String toString(Iterator<?> iterator) {
        StringBuilder sb = new StringBuilder().append('[');
        boolean first = true;
        while (iterator.hasNext()) {
            if (!first) {
                sb.append(", ");
            }
            first = false;
            sb.append(iterator.next());
        }
        return sb.append(']').toString();
    }

    /**
     * Returns the single element contained in {@code iterator}.
     *
     * @throws NoSuchElementException if the iterator is empty
     * @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
     *         iterator is unspecified.
     */
    @CanIgnoreReturnValue // TODO(kak): Consider removing this?
    public static <T> T getOnlyElement(Iterator<T> iterator) {
        T first = iterator.next();
        if (!iterator.hasNext()) {
            return first;
        }

        StringBuilder sb = new StringBuilder().append("expected one element but was: <").append(first);
        for (int i = 0; i < 4 && iterator.hasNext(); i++) {
            sb.append(", ").append(iterator.next());
        }
        if (iterator.hasNext()) {
            sb.append(", ...");
        }
        sb.append('>');

        throw new IllegalArgumentException(sb.toString());
    }

    /**
     * Returns the single element contained in {@code iterator}, or {@code
     * defaultValue} if the iterator is empty.
     *
     * @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
     *         iterator is unspecified.
     */
    @CanIgnoreReturnValue // TODO(kak): Consider removing this?
    @Nullable
    public static <T> T getOnlyElement(Iterator<? extends T> iterator, @Nullable T defaultValue) {
        return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
    }

    /**
     * Copies an iterator's elements into an array. The iterator will be left exhausted: its
     * {@code hasNext()} method will return {@code false}.
     *
     * @param iterator the iterator to copy
     * @param type the type of the elements
     * @return a newly-allocated array into which all the elements of the iterator have been copied
     */
    @GwtIncompatible // Array.newInstance(Class, int)
    public static <T> T[] toArray(Iterator<? extends T> iterator, Class<T> type) {
        List<T> list = Lists.newArrayList(iterator);
        return Iterables.toArray(list, type);
    }

    /**
     * Adds all elements in {@code iterator} to {@code collection}. The iterator will be left
     * exhausted: its {@code hasNext()} method will return {@code false}.
     *
     * @return {@code true} if {@code collection} was modified as a result of this operation
     */
    @CanIgnoreReturnValue
    public static <T> boolean addAll(Collection<T> addTo, Iterator<? extends T> iterator) {
        checkNotNull(addTo);
        checkNotNull(iterator);
        boolean wasModified = false;
        while (iterator.hasNext()) {
            wasModified |= addTo.add(iterator.next());
        }
        return wasModified;
    }

    /**
     * Returns the number of elements in the specified iterator that equal the specified object. The
     * iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
     *
     * @see Collections#frequency
     */
    public static int frequency(Iterator<?> iterator, @Nullable Object element) {
        return size(filter(iterator, equalTo(element)));
    }

    /**
     * Returns an iterator that cycles indefinitely over the elements of {@code
     * iterable}.
     *
     * <p>
     * The returned iterator supports {@code remove()} if the provided iterator does. After
     * {@code remove()} is called, subsequent cycles omit the removed element, which is no longer in
     * {@code iterable}. The iterator's {@code hasNext()} method returns {@code true} until
     * {@code iterable} is empty.
     *
     * <p>
     * <b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
     * should use an explicit {@code break} or be certain that you will eventually remove all the
     * elements.
     */
    public static <T> Iterator<T> cycle(final Iterable<T> iterable) {
        checkNotNull(iterable);
        return new Iterator<T>() {
            Iterator<T> iterator = emptyModifiableIterator();

            @Override
            public boolean hasNext() {
                /*
                 * Don't store a new Iterator until we know the user can't remove() the last
                 * returned element anymore. Otherwise, when we remove from the old iterator, we may
                 * be invalidating the new one. The result is a ConcurrentModificationException or
                 * other bad behavior.
                 *
                 * (If we decide that we really, really hate allocating two Iterators per cycle
                 * instead of one, we can optimistically store the new Iterator and then be willing
                 * to throw it out if the user calls remove().)
                 */
                return iterator.hasNext() || iterable.iterator().hasNext();
            }

            @Override
            public T next() {
                if (!iterator.hasNext()) {
                    iterator = iterable.iterator();
                    if (!iterator.hasNext()) {
                        throw new NoSuchElementException();
                    }
                }
                return iterator.next();
            }

            @Override
            public void remove() {
                iterator.remove();
            }
        };
    }

    /**
     * Returns an iterator that cycles indefinitely over the provided elements.
     *
     * <p>
     * The returned iterator supports {@code remove()}. After {@code remove()} is called, subsequent
     * cycles omit the removed element, but {@code elements} does not change. The iterator's
     * {@code hasNext()} method returns {@code true} until all of the original elements have been
     * removed.
     *
     * <p>
     * <b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
     * should use an explicit {@code break} or be certain that you will eventually remove all the
     * elements.
     */
    @SafeVarargs
    public static <T> Iterator<T> cycle(T... elements) {
        return cycle(Lists.newArrayList(elements));
    }

    /**
     * Combines two iterators into a single iterator. The returned iterator iterates across the
     * elements in {@code a}, followed by the elements in {@code b}. The source iterators are not
     * polled until necessary.
     *
     * <p>
     * The returned iterator supports {@code remove()} when the corresponding input iterator
     * supports it.
     */
    public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b) {
        checkNotNull(a);
        checkNotNull(b);
        return concat(new ConsumingQueueIterator<Iterator<? extends T>>(a, b));
    }

    /**
     * Combines three iterators into a single iterator. The returned iterator iterates across the
     * elements in {@code a}, followed by the elements in {@code b}, followed by the elements in
     * {@code c}. The source iterators are not polled until necessary.
     *
     * <p>
     * The returned iterator supports {@code remove()} when the corresponding input iterator
     * supports it.
     */
    public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c) {
        checkNotNull(a);
        checkNotNull(b);
        checkNotNull(c);
        return concat(new ConsumingQueueIterator<Iterator<? extends T>>(a, b, c));
    }

    /**
     * Combines four iterators into a single iterator. The returned iterator iterates across the
     * elements in {@code a}, followed by the elements in {@code b}, followed by the elements in
     * {@code c}, followed by the elements in {@code d}. The source iterators are not polled until
     * necessary.
     *
     * <p>
     * The returned iterator supports {@code remove()} when the corresponding input iterator
     * supports it.
     */
    public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c,
            Iterator<? extends T> d) {
        checkNotNull(a);
        checkNotNull(b);
        checkNotNull(c);
        checkNotNull(d);
        return concat(new ConsumingQueueIterator<Iterator<? extends T>>(a, b, c, d));
    }

    /**
     * Combines multiple iterators into a single iterator. The returned iterator iterates across the
     * elements of each iterator in {@code inputs}. The input iterators are not polled until
     * necessary.
     *
     * <p>
     * The returned iterator supports {@code remove()} when the corresponding input iterator
     * supports it.
     *
     * @throws NullPointerException if any of the provided iterators is null
     */
    public static <T> Iterator<T> concat(Iterator<? extends T>... inputs) {
        for (Iterator<? extends T> input : checkNotNull(inputs)) {
            checkNotNull(input);
        }
        return concat(new ConsumingQueueIterator<Iterator<? extends T>>(inputs));
    }

    /**
     * Combines multiple iterators into a single iterator. The returned iterator iterates across the
     * elements of each iterator in {@code inputs}. The input iterators are not polled until
     * necessary.
     *
     * <p>
     * The returned iterator supports {@code remove()} when the corresponding input iterator
     * supports it. The methods of the returned iterator may throw {@code NullPointerException} if
     * any of the input iterators is null.
     */
    public static <T> Iterator<T> concat(Iterator<? extends Iterator<? extends T>> inputs) {
        return new ConcatenatedIterator<T>(inputs);
    }

    /**
     * Divides an iterator into unmodifiable sublists of the given size (the final list may be
     * smaller). For example, partitioning an iterator containing {@code [a, b, c, d, e]} with a
     * partition size of 3 yields {@code
     * [[a, b, c], [d, e]]} -- an outer iterator containing two inner lists of three and two
     * elements, all in the original order.
     *
     * <p>
     * The returned lists implement {@link java.util.RandomAccess}.
     *
     * @param iterator the iterator to return a partitioned view of
     * @param size the desired size of each partition (the last may be smaller)
     * @return an iterator of immutable lists containing the elements of {@code
     *     iterator} divided into partitions
     * @throws IllegalArgumentException if {@code size} is nonpositive
     */
    public static <T> UnmodifiableIterator<List<T>> partition(Iterator<T> iterator, int size) {
        return partitionImpl(iterator, size, false);
    }

    /**
     * Divides an iterator into unmodifiable sublists of the given size, padding the final iterator
     * with null values if necessary. For example, partitioning an iterator containing
     * {@code [a, b, c, d, e]} with a partition size of 3 yields {@code [[a, b, c], [d, e, null]]}
     * -- an outer iterator containing two inner lists of three elements each, all in the original
     * order.
     *
     * <p>
     * The returned lists implement {@link java.util.RandomAccess}.
     *
     * @param iterator the iterator to return a partitioned view of
     * @param size the desired size of each partition
     * @return an iterator of immutable lists containing the elements of {@code
     *     iterator} divided into partitions (the final iterable may have trailing null elements)
     * @throws IllegalArgumentException if {@code size} is nonpositive
     */
    public static <T> UnmodifiableIterator<List<T>> paddedPartition(Iterator<T> iterator, int size) {
        return partitionImpl(iterator, size, true);
    }

    private static <T> UnmodifiableIterator<List<T>> partitionImpl(final Iterator<T> iterator, final int size,
            final boolean pad) {
        checkNotNull(iterator);
        checkArgument(size > 0);
        return new UnmodifiableIterator<List<T>>() {
            @Override
            public boolean hasNext() {
                return iterator.hasNext();
            }

            @Override
            public List<T> next() {
                if (!hasNext()) {
                    throw new NoSuchElementException();
                }
                Object[] array = new Object[size];
                int count = 0;
                for (; count < size && iterator.hasNext(); count++) {
                    array[count] = iterator.next();
                }
                for (int i = count; i < size; i++) {
                    array[i] = null; // for GWT
                }

                @SuppressWarnings("unchecked") // we only put Ts in it
                List<T> list = Collections.unmodifiableList((List<T>) Arrays.asList(array));
                return (pad || count == size) ? list : list.subList(0, count);
            }
        };
    }

    /**
     * Returns a view of {@code unfiltered} containing all elements that satisfy the input predicate
     * {@code retainIfTrue}.
     */
    public static <T> UnmodifiableIterator<T> filter(final Iterator<T> unfiltered,
            final Predicate<? super T> retainIfTrue) {
        checkNotNull(unfiltered);
        checkNotNull(retainIfTrue);
        return new AbstractIterator<T>() {
            @Override
            protected T computeNext() {
                while (unfiltered.hasNext()) {
                    T element = unfiltered.next();
                    if (retainIfTrue.apply(element)) {
                        return element;
                    }
                }
                return endOfData();
            }
        };
    }

    /**
     * Returns a view of {@code unfiltered} containing all elements that are of the type
     * {@code desiredType}.
     */
    @SuppressWarnings("unchecked") // can cast to <T> because non-Ts are removed
    @GwtIncompatible // Class.isInstance
    public static <T> UnmodifiableIterator<T> filter(Iterator<?> unfiltered, Class<T> desiredType) {
        return (UnmodifiableIterator<T>) filter(unfiltered, instanceOf(desiredType));
    }

    /**
     * Returns {@code true} if one or more elements returned by {@code iterator} satisfy the given
     * predicate.
     */
    public static <T> boolean any(Iterator<T> iterator, Predicate<? super T> predicate) {
        return indexOf(iterator, predicate) != -1;
    }

    /**
     * Returns {@code true} if every element returned by {@code iterator} satisfies the given
     * predicate. If {@code iterator} is empty, {@code true} is returned.
     */
    public static <T> boolean all(Iterator<T> iterator, Predicate<? super T> predicate) {
        checkNotNull(predicate);
        while (iterator.hasNext()) {
            T element = iterator.next();
            if (!predicate.apply(element)) {
                return false;
            }
        }
        return true;
    }

    /**
     * Returns the first element in {@code iterator} that satisfies the given predicate; use this
     * method only when such an element is known to exist. If no such element is found, the iterator
     * will be left exhausted: its {@code
     * hasNext()} method will return {@code false}. If it is possible that <i>no</i> element will
     * match, use {@link #tryFind} or {@link #find(Iterator, Predicate, Object)} instead.
     *
     * @throws NoSuchElementException if no element in {@code iterator} matches the given predicate
     */
    public static <T> T find(Iterator<T> iterator, Predicate<? super T> predicate) {
        return filter(iterator, predicate).next();
    }

    /**
     * Returns the first element in {@code iterator} that satisfies the given predicate. If no such
     * element is found, {@code defaultValue} will be returned from this method and the iterator
     * will be left exhausted: its {@code hasNext()} method will return {@code false}. Note that
     * this can usually be handled more naturally using {@code
     * tryFind(iterator, predicate).or(defaultValue)}.
     *
     * @since 7.0
     */
    @Nullable
    public static <T> T find(Iterator<? extends T> iterator, Predicate<? super T> predicate, @Nullable T defaultValue) {
        return getNext(filter(iterator, predicate), defaultValue);
    }

    /**
     * Returns an {@link Optional} containing the first element in {@code
     * iterator} that satisfies the given predicate, if such an element exists. If no such element
     * is found, an empty {@link Optional} will be returned from this method and the iterator will
     * be left exhausted: its {@code
     * hasNext()} method will return {@code false}.
     *
     * <p>
     * <b>Warning:</b> avoid using a {@code predicate} that matches {@code
     * null}. If {@code null} is matched in {@code iterator}, a NullPointerException will be thrown.
     *
     * @since 11.0
     */
    public static <T> Optional<T> tryFind(Iterator<T> iterator, Predicate<? super T> predicate) {
        UnmodifiableIterator<T> filteredIterator = filter(iterator, predicate);
        return filteredIterator.hasNext() ? Optional.of(filteredIterator.next()) : Optional.<T>absent();
    }

    /**
     * Returns the index in {@code iterator} of the first element that satisfies the provided
     * {@code predicate}, or {@code -1} if the Iterator has no such elements.
     *
     * <p>
     * More formally, returns the lowest index {@code i} such that
     * {@code predicate.apply(Iterators.get(iterator, i))} returns {@code true}, or {@code -1} if
     * there is no such index.
     *
     * <p>
     * If -1 is returned, the iterator will be left exhausted: its {@code hasNext()} method will
     * return {@code false}. Otherwise, the iterator will be set to the element which satisfies the
     * {@code predicate}.
     *
     * @since 2.0
     */
    public static <T> int indexOf(Iterator<T> iterator, Predicate<? super T> predicate) {
        checkNotNull(predicate, "predicate");
        for (int i = 0; iterator.hasNext(); i++) {
            T current = iterator.next();
            if (predicate.apply(current)) {
                return i;
            }
        }
        return -1;
    }

    /**
     * Returns a view containing the result of applying {@code function} to each element of
     * {@code fromIterator}.
     *
     * <p>
     * The returned iterator supports {@code remove()} if {@code fromIterator} does. After a
     * successful {@code remove()} call, {@code fromIterator} no longer contains the corresponding
     * element.
     */
    public static <F, T> Iterator<T> transform(final Iterator<F> fromIterator,
            final Function<? super F, ? extends T> function) {
        checkNotNull(function);
        return new TransformedIterator<F, T>(fromIterator) {
            @Override
            T transform(F from) {
                return function.apply(from);
            }
        };
    }

    /**
     * Advances {@code iterator} {@code position + 1} times, returning the element at the
     * {@code position}th position.
     *
     * @param position position of the element to return
     * @return the element at the specified position in {@code iterator}
     * @throws IndexOutOfBoundsException if {@code position} is negative or greater than or equal to
     *         the number of elements remaining in {@code iterator}
     */
    public static <T> T get(Iterator<T> iterator, int position) {
        checkNonnegative(position);
        int skipped = advance(iterator, position);
        if (!iterator.hasNext()) {
            throw new IndexOutOfBoundsException("position (" + position
                    + ") must be less than the number of elements that remained (" + skipped + ")");
        }
        return iterator.next();
    }

    static void checkNonnegative(int position) {
        if (position < 0) {
            throw new IndexOutOfBoundsException("position (" + position + ") must not be negative");
        }
    }

    /**
     * Advances {@code iterator} {@code position + 1} times, returning the element at the
     * {@code position}th position or {@code defaultValue} otherwise.
     *
     * @param position position of the element to return
     * @param defaultValue the default value to return if the iterator is empty or if
     *        {@code position} is greater than the number of elements remaining in {@code iterator}
     * @return the element at the specified position in {@code iterator} or {@code defaultValue} if
     *         {@code iterator} produces fewer than {@code position + 1} elements.
     * @throws IndexOutOfBoundsException if {@code position} is negative
     * @since 4.0
     */
    @Nullable
    public static <T> T get(Iterator<? extends T> iterator, int position, @Nullable T defaultValue) {
        checkNonnegative(position);
        advance(iterator, position);
        return getNext(iterator, defaultValue);
    }

    /**
     * Returns the next element in {@code iterator} or {@code defaultValue} if the iterator is
     * empty. The {@link Iterables} analog to this method is {@link Iterables#getFirst}.
     *
     * @param defaultValue the default value to return if the iterator is empty
     * @return the next element of {@code iterator} or the default value
     * @since 7.0
     */
    @Nullable
    public static <T> T getNext(Iterator<? extends T> iterator, @Nullable T defaultValue) {
        return iterator.hasNext() ? iterator.next() : defaultValue;
    }

    /**
     * Advances {@code iterator} to the end, returning the last element.
     *
     * @return the last element of {@code iterator}
     * @throws NoSuchElementException if the iterator is empty
     */
    public static <T> T getLast(Iterator<T> iterator) {
        while (true) {
            T current = iterator.next();
            if (!iterator.hasNext()) {
                return current;
            }
        }
    }

    /**
     * Advances {@code iterator} to the end, returning the last element or {@code defaultValue} if
     * the iterator is empty.
     *
     * @param defaultValue the default value to return if the iterator is empty
     * @return the last element of {@code iterator}
     * @since 3.0
     */
    @Nullable
    public static <T> T getLast(Iterator<? extends T> iterator, @Nullable T defaultValue) {
        return iterator.hasNext() ? getLast(iterator) : defaultValue;
    }

    /**
     * Calls {@code next()} on {@code iterator}, either {@code numberToAdvance} times or until
     * {@code hasNext()} returns {@code false}, whichever comes first.
     *
     * @return the number of elements the iterator was advanced
     * @since 13.0 (since 3.0 as {@code Iterators.skip})
     */
    @CanIgnoreReturnValue
    public static int advance(Iterator<?> iterator, int numberToAdvance) {
        checkNotNull(iterator);
        checkArgument(numberToAdvance >= 0, "numberToAdvance must be nonnegative");

        int i;
        for (i = 0; i < numberToAdvance && iterator.hasNext(); i++) {
            iterator.next();
        }
        return i;
    }

    /**
     * Returns a view containing the first {@code limitSize} elements of {@code
     * iterator}. If {@code iterator} contains fewer than {@code limitSize} elements, the returned
     * view contains all of its elements. The returned iterator supports {@code remove()} if
     * {@code iterator} does.
     *
     * @param iterator the iterator to limit
     * @param limitSize the maximum number of elements in the returned iterator
     * @throws IllegalArgumentException if {@code limitSize} is negative
     * @since 3.0
     */
    public static <T> Iterator<T> limit(final Iterator<T> iterator, final int limitSize) {
        checkNotNull(iterator);
        checkArgument(limitSize >= 0, "limit is negative");
        return new Iterator<T>() {
            private int count;

            @Override
            public boolean hasNext() {
                return count < limitSize && iterator.hasNext();
            }

            @Override
            public T next() {
                if (!hasNext()) {
                    throw new NoSuchElementException();
                }
                count++;
                return iterator.next();
            }

            @Override
            public void remove() {
                iterator.remove();
            }
        };
    }

    /**
     * Returns a view of the supplied {@code iterator} that removes each element from the supplied
     * {@code iterator} as it is returned.
     *
     * <p>
     * The provided iterator must support {@link Iterator#remove()} or else the returned iterator
     * will fail on the first call to {@code
     * next}.
     *
     * @param iterator the iterator to remove and return elements from
     * @return an iterator that removes and returns elements from the supplied iterator
     * @since 2.0
     */
    public static <T> Iterator<T> consumingIterator(final Iterator<T> iterator) {
        checkNotNull(iterator);
        return new UnmodifiableIterator<T>() {
            @Override
            public boolean hasNext() {
                return iterator.hasNext();
            }

            @Override
            public T next() {
                T next = iterator.next();
                iterator.remove();
                return next;
            }

            @Override
            public String toString() {
                return "Iterators.consumingIterator(...)";
            }
        };
    }

    /**
     * Deletes and returns the next value from the iterator, or returns {@code null} if there is no
     * such value.
     */
    @Nullable
    static <T> T pollNext(Iterator<T> iterator) {
        if (iterator.hasNext()) {
            T result = iterator.next();
            iterator.remove();
            return result;
        } else {
            return null;
        }
    }

    // Methods only in Iterators, not in Iterables

    /**
     * Clears the iterator using its remove method.
     */
    static void clear(Iterator<?> iterator) {
        checkNotNull(iterator);
        while (iterator.hasNext()) {
            iterator.next();
            iterator.remove();
        }
    }

    /**
     * Returns an iterator containing the elements of {@code array} in order. The returned iterator
     * is a view of the array; subsequent changes to the array will be reflected in the iterator.
     *
     * <p>
     * <b>Note:</b> It is often preferable to represent your data using a collection type, for
     * example using {@link Arrays#asList(Object[])}, making this method unnecessary.
     *
     * <p>
     * The {@code Iterable} equivalent of this method is either {@link Arrays#asList(Object[])},
     * {@link ImmutableList#copyOf(Object[])}}, or {@link ImmutableList#of}.
     */
    @SafeVarargs
    public static <T> UnmodifiableIterator<T> forArray(final T... array) {
        return forArray(array, 0, array.length, 0);
    }

    private static final class ArrayItr<T> extends AbstractIndexedListIterator<T> {
        static final UnmodifiableListIterator<Object> EMPTY = new ArrayItr<Object>(new Object[0], 0, 0, 0);

        private final T[] array;
        private final int offset;

        ArrayItr(T[] array, int offset, int length, int index) {
            super(length, index);
            this.array = array;
            this.offset = offset;
        }

        @Override
        protected T get(int index) {
            return array[offset + index];
        }
    }

    /**
     * Returns a list iterator containing the elements in the specified range of {@code array} in
     * order, starting at the specified index.
     *
     * <p>
     * The {@code Iterable} equivalent of this method is {@code
     * Arrays.asList(array).subList(offset, offset + length).listIterator(index)}.
     */
    static <T> UnmodifiableListIterator<T> forArray(final T[] array, final int offset, int length, int index) {
        checkArgument(length >= 0);
        int end = offset + length;

        // Technically we should give a slightly more descriptive error on overflow
        Preconditions.checkPositionIndexes(offset, end, array.length);
        Preconditions.checkPositionIndex(index, length);
        if (length == 0) {
            return emptyListIterator();
        }
        return new ArrayItr<T>(array, offset, length, index);
    }

    /**
     * Returns an iterator containing only {@code value}.
     *
     * <p>
     * The {@link Iterable} equivalent of this method is {@link Collections#singleton}.
     */
    public static <T> UnmodifiableIterator<T> singletonIterator(@Nullable final T value) {
        return new UnmodifiableIterator<T>() {
            boolean done;

            @Override
            public boolean hasNext() {
                return !done;
            }

            @Override
            public T next() {
                if (done) {
                    throw new NoSuchElementException();
                }
                done = true;
                return value;
            }
        };
    }

    /**
     * Adapts an {@code Enumeration} to the {@code Iterator} interface.
     *
     * <p>
     * This method has no equivalent in {@link Iterables} because viewing an {@code Enumeration} as
     * an {@code Iterable} is impossible. However, the contents can be <i>copied</i> into a
     * collection using {@link Collections#list}.
     */
    public static <T> UnmodifiableIterator<T> forEnumeration(final Enumeration<T> enumeration) {
        checkNotNull(enumeration);
        return new UnmodifiableIterator<T>() {
            @Override
            public boolean hasNext() {
                return enumeration.hasMoreElements();
            }

            @Override
            public T next() {
                return enumeration.nextElement();
            }
        };
    }

    /**
     * Adapts an {@code Iterator} to the {@code Enumeration} interface.
     *
     * <p>
     * The {@code Iterable} equivalent of this method is either {@link Collections#enumeration} (if
     * you have a {@link Collection}), or {@code Iterators.asEnumeration(collection.iterator())}.
     */
    public static <T> Enumeration<T> asEnumeration(final Iterator<T> iterator) {
        checkNotNull(iterator);
        return new Enumeration<T>() {
            @Override
            public boolean hasMoreElements() {
                return iterator.hasNext();
            }

            @Override
            public T nextElement() {
                return iterator.next();
            }
        };
    }

    /**
     * Implementation of PeekingIterator that avoids peeking unless necessary.
     */
    private static class PeekingImpl<E> implements PeekingIterator<E> {

        private final Iterator<? extends E> iterator;
        private boolean hasPeeked;
        private E peekedElement;

        public PeekingImpl(Iterator<? extends E> iterator) {
            this.iterator = checkNotNull(iterator);
        }

        @Override
        public boolean hasNext() {
            return hasPeeked || iterator.hasNext();
        }

        @Override
        public E next() {
            if (!hasPeeked) {
                return iterator.next();
            }
            E result = peekedElement;
            hasPeeked = false;
            peekedElement = null;
            return result;
        }

        @Override
        public void remove() {
            checkState(!hasPeeked, "Can't remove after you've peeked at next");
            iterator.remove();
        }

        @Override
        public E peek() {
            if (!hasPeeked) {
                peekedElement = iterator.next();
                hasPeeked = true;
            }
            return peekedElement;
        }
    }

    /**
     * Returns a {@code PeekingIterator} backed by the given iterator.
     *
     * <p>
     * Calls to the {@code peek} method with no intervening calls to {@code
     * next} do not affect the iteration, and hence return the same object each time. A subsequent
     * call to {@code next} is guaranteed to return the same object again. For example:
     * 
     * <pre>
     *    {@code
     *
     *   PeekingIterator<String> peekingIterator =
     *       Iterators.peekingIterator(Iterators.forArray("a", "b"));
     *   String a1 = peekingIterator.peek(); // returns "a"
     *   String a2 = peekingIterator.peek(); // also returns "a"
     *   String a3 = peekingIterator.next(); // also returns "a"}
     * </pre>
     *
     * <p>
     * Any structural changes to the underlying iteration (aside from those performed by the
     * iterator's own {@link PeekingIterator#remove()} method) will leave the iterator in an
     * undefined state.
     *
     * <p>
     * The returned iterator does not support removal after peeking, as explained by
     * {@link PeekingIterator#remove()}.
     *
     * <p>
     * Note: If the given iterator is already a {@code PeekingIterator}, it <i>might</i> be returned
     * to the caller, although this is neither guaranteed to occur nor required to be consistent.
     * For example, this method <i>might</i> choose to pass through recognized implementations of
     * {@code PeekingIterator} when the behavior of the implementation is known to meet the contract
     * guaranteed by this method.
     *
     * <p>
     * There is no {@link Iterable} equivalent to this method, so use this method to wrap each
     * individual iterator as it is generated.
     *
     * @param iterator the backing iterator. The {@link PeekingIterator} assumes ownership of this
     *        iterator, so users should cease making direct calls to it after calling this method.
     * @return a peeking iterator backed by that iterator. Apart from the additional
     *         {@link PeekingIterator#peek()} method, this iterator behaves exactly the same as
     *         {@code iterator}.
     */
    public static <T> PeekingIterator<T> peekingIterator(Iterator<? extends T> iterator) {
        if (iterator instanceof PeekingImpl) {
            // Safe to cast <? extends T> to <T> because PeekingImpl only uses T
            // covariantly (and cannot be subclassed to add non-covariant uses).
            @SuppressWarnings("unchecked")
            PeekingImpl<T> peeking = (PeekingImpl<T>) iterator;
            return peeking;
        }
        return new PeekingImpl<T>(iterator);
    }

    /**
     * Simply returns its argument.
     *
     * @deprecated no need to use this
     * @since 10.0
     */
    @Deprecated
    public static <T> PeekingIterator<T> peekingIterator(PeekingIterator<T> iterator) {
        return checkNotNull(iterator);
    }

    /**
     * Returns an iterator over the merged contents of all given {@code iterators}, traversing every
     * element of the input iterators. Equivalent entries will not be de-duplicated.
     *
     * <p>
     * Callers must ensure that the source {@code iterators} are in non-descending order as this
     * method does not sort its input.
     *
     * <p>
     * For any equivalent elements across all {@code iterators}, it is undefined which element is
     * returned first.
     *
     * @since 11.0
     */
    @Beta
    public static <T> UnmodifiableIterator<T> mergeSorted(Iterable<? extends Iterator<? extends T>> iterators,
            Comparator<? super T> comparator) {
        checkNotNull(iterators, "iterators");
        checkNotNull(comparator, "comparator");

        return new MergingIterator<T>(iterators, comparator);
    }

    /**
     * An iterator that performs a lazy N-way merge, calculating the next value each time the
     * iterator is polled. This amortizes the sorting cost over the iteration and requires less
     * memory than sorting all elements at once.
     *
     * <p>
     * Retrieving a single element takes approximately O(log(M)) time, where M is the number of
     * iterators. (Retrieving all elements takes approximately O(N*log(M)) time, where N is the
     * total number of elements.)
     */
    private static class MergingIterator<T> extends UnmodifiableIterator<T> {
        final Queue<PeekingIterator<T>> queue;

        public MergingIterator(Iterable<? extends Iterator<? extends T>> iterators,
                final Comparator<? super T> itemComparator) {
            // A comparator that's used by the heap, allowing the heap
            // to be sorted based on the top of each iterator.
            Comparator<PeekingIterator<T>> heapComparator = new Comparator<PeekingIterator<T>>() {
                @Override
                public int compare(PeekingIterator<T> o1, PeekingIterator<T> o2) {
                    return itemComparator.compare(o1.peek(), o2.peek());
                }
            };

            queue = new PriorityQueue<PeekingIterator<T>>(2, heapComparator);

            for (Iterator<? extends T> iterator : iterators) {
                if (iterator.hasNext()) {
                    queue.add(Iterators.peekingIterator(iterator));
                }
            }
        }

        @Override
        public boolean hasNext() {
            return !queue.isEmpty();
        }

        @Override
        public T next() {
            PeekingIterator<T> nextIter = queue.remove();
            T next = nextIter.next();
            if (nextIter.hasNext()) {
                queue.add(nextIter);
            }
            return next;
        }
    }

    private static class ConcatenatedIterator<T> extends MultitransformedIterator<Iterator<? extends T>, T> {

        public ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> iterators) {
            super(getComponentIterators(iterators));
        }

        @Override
        Iterator<? extends T> transform(Iterator<? extends T> iterator) {
            return iterator;
        }

        /**
         * Using the component iterators, rather than the input iterators directly, allows for
         * higher performance in the case of nested concatenation.
         */
        private static <T> Iterator<Iterator<? extends T>> getComponentIterators(
                Iterator<? extends Iterator<? extends T>> iterators) {
            return new MultitransformedIterator<Iterator<? extends T>, Iterator<? extends T>>(iterators) {
                @Override
                Iterator<? extends Iterator<? extends T>> transform(Iterator<? extends T> iterator) {
                    if (iterator instanceof ConcatenatedIterator) {
                        ConcatenatedIterator<? extends T> concatIterator = (ConcatenatedIterator<? extends T>) iterator;
                        return getComponentIterators(concatIterator.backingIterator);
                    } else {
                        return Iterators.singletonIterator(iterator);
                    }
                }
            };
        }
    }

    /**
     * Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557
     */
    static <T> ListIterator<T> cast(Iterator<T> iterator) {
        return (ListIterator<T>) iterator;
    }
}
